This research will test the hypothesis that drugs exert rate-dependent effects on behavior due to their stimulus properties. The stimulus properties of drugs might also be responsible for the decrements in stimulus control which several drugs produce. D-amphetamine, which has been show to produce rate-dependent effects on fixed interval (FI) performance, will be administered chronically to rats performing on FI. These animals will then be switched to the nondrug state to see if such a stimulus change will also produce a rate-dependent alteration in performance. A loud tone stimulus will be tested for its effects on FI responding and on a visual discrimination performance and those effects compared to the effects of various doses of scopolamine. Several drugs will be studied to see if rats can easily distinguish between drug and nondrug states, and at which doses. The same doses of these drugs will be given to rats performing on FI schedules and to rats performing a visual discrimination to see if the stimulus properties of the drugs correlate with the rate-dependence producing and the stimulus control disrupting effects of the drugs.